1. FIELD OF THE INVENTION
The present invention is concerned with techniques which can be used to produce multilayered ceramic structures containing copper-based conductors.
2. BACKGROUND OF THE INVENTION
Multilayered glass-ceramic structures are used in the production of electronic substrates and devices. Many different types of structures can be used, and a few of these structures are described below. For example, a multilayered ceramic circuit substrate may comprise metal layers which act as patterned electrical conductors sandwiched between ceramic layers which act as insulators. The substrates may be designed with termination pads for attaching semiconductor chips, connector leads, capacitors, resistors, covers, etc. Interconnection between buried conductor levels can be achieved through so-called "vias" formed by metal paste filled holes in the individual glass-ceramic layers formed prior to lamination, which, upon sintering, will become a sintered dense metal interconnection of copper-based conductor.
In general, conventional ceramic structures are formed from ceramic green sheets which are prepared by mixing a ceramic particulate, a thermoplastic polymer binder, and solvents. This composition is spread or cast into ceramic sheets or slips from which the solvents are subsequently volatilized to provide coherent and self-supporting flexible green sheet. The green sheet is eventually fired at temperatures sufficient to drive off the polymeric binder resin and sinter the ceramic particulates together into a densified ceramic substrate.
The electrical conductors used in formation of the electronic substrate may be high melting point metals such as molybdenum and tungsten or a noble metal such as gold. However, it is more desirable to use a conductor having a low electrical resistance and low cost, such as copper and alloys thereof.
Use of copper-based conductors in the multilayered structures requires the use of process techniques which do not oxidize the copper during the removal of binder resin and solvents, and sintering of the ceramic particulates together into the densified ceramic substrate. Many of the methods proposed for making the multilayered glass-ceramic structures call for firing of the green sheet in an inert atmosphere such as nitrogen, to avoid oxidizing the copper-based conductors in the structure. This frequently results in incomplete decomposition of the binder resin or the formation of carbonaceous residue within the structure, thereby deteriorating the mechanical strength and dielectric properties (including electrical insulation) of the structure.
U.S. Pat. No. 4,234,367 to Lester W. Herron et al. discloses a method for forming sintered glass-ceramic substrates containing multilevel, interconnected thick film circuit patterns of copper-based conductors. These substrates are obtained by firing the glass-ceramic in a controlled ambient of hydrogen and water at temperatures below the melting point of copper. The controlled ambient as defined by Herron et al. is slightly reducing to copper but highly oxidizing to carbon at all times, so the resin binder can be removed while producing substrates with non-oxidized metallic copper.
U.S. Pat. No. 4,079,156 to Youtsey et al. discloses a method of preventing oxidation of the non-noble conductive metal, wherein the metal is alloyed with at least one oxidizable material which is preferentially oxidized during the firing of thick film electronic components in an oxidative atmosphere.
U.S. Pat. No. 4,474,731 to Brownlow et al. discloses a process for the removal of carbon residues during sintering of ceramics wherein a pyrolysis catalyst is utilized to permit binder resin removal in a low oxygen atmosphere without the accumulation of carbonaceous residue. In the case of copper, the pyrolysis ambient is disclosed as containing hydrogen and water vapor ratios within a specified range in order to minimize the oxidation of the copper.
U.S. Pat. No. 4,504,339 to Kamehara et al. discloses a method for producing a multilayered glass-ceramic structure with copper-based conductors therein, wherein the multilayered structure is fired in an inert atmosphere containing water vapor, the partial pressure of which is from 0.005 to 0.3 atmosphere.
U.S. Pat. No. 4,517,155 to Prakash et al. discloses a method of producing copper end terminations on multielectrode ceramic capacitors, wherein the capacitor is fired in an atmosphere of nitrogen which contains a controlled partial pressure of oxygen. The recommended atmosphere, during burn-out of the organic binders contained in the structure, is comprised of nitrogen containing from between 20 and 200 ppm of oxygen. About 50-150 ppm of oxygen is preferred during the initial burn-out step. Higher partial pressures of oxygen are said to lead to oxidation of the copper constituent, resulting in increased resistivity and poor solderability of the fired termination.
Thus, the patents described above depend on the use of neutral ambients, control of ambient oxygen content to about 200 ppm or less, or alloying of the copper with an oxygen scavenger to prevent copper oxidation during glass ceramic or ceramic binder/vehicle burn out. It would be desirable to have a method of producing multilayered ceramic structures with copper-based conductors which permits the use of ambients containing oxygen contents above the 200 ppm level, to permit the rapid removal of binder resin at lower temperatures via oxidative degradation. It would be desirable to have a method of producing such structures in an ambient which does not contain water vapor, since the presence of water vapor causes bloating and porosity of the coalesced glass-ceramic/ copper laminate if process conditions are not carefully controlled. In addition, typically the onset of sintering of a copper-based conductor takes place at temperatures ranging from about 250.degree. C. to 400.degree. C., whereas sintering of the ceramic takes place at about 800.degree. C. This difference in sintering temperature presents dimensional control problems during processing of the multilayer structure. It would be desirable to have a method of increasing the temperature at which the copper-based metallurgy sinters, so that it more nearly matches that at which the ceramic sinters.